The overall objective of our research project is to elucidate further the influences of thyroid hormones on skeletal muscle growth in chickens. Our studies will focus on processes which precede and underlie the nearly two-fold increase in muscle weights between 17 and 20 days of embryogenesis, which are prevented by goitrogen treatment. The goals of our proposed research for the coming year are the following: 1. Study of thyroid hormone action. Further experiments will be done to confirm and extend our earlier observations of ribosomal-polysomal sucrose density gradient sedimentation patterns for extracts of embryonic leg muscles. Changes in the patterns following treatment with the goitrogens methimazole and perchlorate and with thyroxine and triiodothyronine replacement regimens will also be studied further. We also plan to investigate changes in the pool of 40S ribosomal subunits and the incorporation of H3-uridine into the subunits during thyroid deprivation and replacement in late embryogenesis. Changes in the activity of RNA polymerases will also be ascertained for nuclei obtained from leg muscles of embryos with different thyroid states. 2. Triiodothyronine (T3) binding to saturable receptor sites. Further studies will be done to assess T3-binding to saturable receptors in liver and muscle nuclei. 3. Studies in dystrophic chickens. Changes in the activity of 6-phosphogluconic acid dehyrogenase in dystrophic muscle will be correlated with changes in nuclear proliferation and lipid deposition. BIBLIOGRAPHIC REFERENCES: A. King, D.B. and King, C.R. 1975. Thyroidal influence on gastrocnemius and sartorius muscle growth in young White Leghorn cockerals. (This paper was subunited to Gen. Comp. Endocrinol. and is currently being revised according to suggestions from the reviewers.) B. King, D.B. and Delfiner, J.S. 1974. Effect of the goitrogen methimazole on skeletal muscle growth of chick embryos. Gen. Comp. Endocrinol. 24:17-27.